File question: the upload button has no accessible name #3839

Describe the bug
The upload button of a file question has no accessible name. Screen readers announce it as "button", and nothing tells the user what it does (WCAG 4.1.2 Name, Role, Value). axe reports it as button-name (critical).

In src/components/Questions/QuestionFile.vue the NcButton next to the hidden file input only has an icon (IconUpload / IconUploadMultiple) and no aria-label. The visible text "Add new file as answer" belongs to the <label> around the hidden <input type="file">, not to the button.

In the editor the button is also disabled (:disabled="!readOnly || …"), so it shows up there as an unnamed, disabled button.

To Reproduce

1. Create a form with a file question.
2. Open the form to answer it (/apps/forms/<hash>/submit).
3. Inspect the upload button with the browser's accessibility tree, or run axe on the page.
4. The button has no name.

Expected behavior
The button has a name, for example :aria-label="t('forms', 'Upload a file')", or 'Upload files' when several files are allowed. Alternatively, the button is hidden from assistive technology when the labelled file input is the real control.

Nextcloud (please complete the following information):

• Nextcloud-Version: master
• Forms-Version: main (5.5.0-dev)

Additional context
Found while reviewing #3123 (conditional questions). It is not caused by that PR: a top-level file question on main has the same problem.